1. Field of the Invention
The invention generally relates to a coherent pulse source of a high power electromagnetic radiation produced by long-range wakefields induced in a slow-wave structure by a specially conditioned weakly relativistic electron beam produced by a photo gun. More particularly, the present invention is directed to providing a coherent high-power terahertz source via resonant Cherenkov radiation of a THz-modulated electron beam.
2. Description of the Prior Art
In the entire spectrum of available electromagnetic sources there is a gap between microwave and far infrared regions, where effective and compact, relatively inexpensive high-power sources are missing. A huge variety of applications in biology, medicine, chemistry, solid state physics, radio astronomy, homeland security, environment monitoring, microelectronics, plasma diagnostics, and industry are anticipating powerful terahertz (THz) sources for middle-size and small labs and businesses. The applications are related to fast processes, emerging time-domain spectroscopy (TDS), and imaging that require short THz pulses of high intensity. A heavy demand for terahertz technology also exists in the communications industry. Development of a powerful THz transmitter will result in a dramatic increase in the available bandwidth in wavelength-division-multiplexed communications networks.
Electron beams with time structures ranging typically from DC (as in electrostatic accelerator columns) to dozen(s) of picoseconds (as in photoinjectors) are capable of producing THz-radiation using e-beam-based or linac-driven sources such as Free Electron Lasers (FELs), Compton backscattering sources, traveling-wave tubes, klinotrons, and Smith-Purcell devices.
Currently, a few FELs are built to operate at THz frequencies. Typically such an FEL is driven by an electron accelerator and contains an undulator and an optical cavity. The first FEL facility to provide THz radiation to users has been the UCSB (University of California, Santa Barbara, Calif.)-FEL (0.3-0.8 mm wavelength). It is driven by a 6-MeV electrostatic accelerator with beam recirculation that delivers up to ˜2 A beam current of relatively high quality (˜10 mm-mrad emittance, and 0.3% energy spread). The maximum pulse power produced is 6 kW; this is short of the expected power of ˜10 kW (in 1-20 μs pulse length) because of mode competition in the overmoded optical cavity (˜5.4 m length) used to generate the radiation.
The largest FEL Facility at JLAB (Thomas Jefferson Laboratory) produces a broadband THz radiation with φW average and ˜1 MW peak power.
To date the Novosibirsk (Russia) FEL is the most powerful coherent THz source operating at 0.12-0.24 mm wavelengths and 0.3% line width to deliver 0.4 kW average power and up to ˜MW peak power and comprises a 20 m long optical cavity, and a long undulator driven by 40-50 MeV e-beam accelerated in a RF linac with energy recovery.
A super-radiant FEL does not have an optical cavity. The ENEA-Frascati FEL-CATS source operates in the 0.4-0.7 THz range with about 10% FWHM line width. The radiation beam has a pulsed structure composed of wave-packets in the 3 to 10 ps range, spaced at a repetition frequency of 3 GHz. A 5-microsecond long train of such pulses (macropulse) is generated and repeated at a rate of a few Hz. The power is 1.5 kW measured in the macropulse at 0.4 THz (corresponding to up to 8 kW peak in each micropulse).
Compact THz sources are basically CW devices of two types: vacuum and solid state. Vacuum devices use a non-relativistic low-power electron beam interacting with micro fabricated surfaces to generate diffraction radiation in an open geometry (e.g. Orotrons, Klinotrons, Smith-Purcell devices), or on a traveling wave in a closed system (e.g. the Backward Wave Oscillator (BWO) or Traveling Wave Tube (TWT). The typical power levels do not exceed a fraction of a Watt at terahertz frequencies. The power is typically limited by a low beam current density and a low degree of modulation occurring in the same limited interaction space.
Solid-state devices are low-power generators (or low-gain amplifiers integrated into a matrix array) based on Schottky varactor, high frequency Gunn, IMPATT or TUNNET diodes. The power produced in such devices is between tens and hundreds of milliwatts.
The most advanced solid-state device is the recently developed 1-4 THz laser based on lightly doped p-type germanium mono-crystals. The maximum emitted power depends on the crystal volume and can range from a few μW to several Watts, with duty cycles of up to 5%. Conventional gas lasers are line-tunable in the range 0.3 to 5 THz (λ=1000 to 60 μm) although with limited power (typically 100 mW for methanol).
Other known THz devices such as Quantum Cascade Lasers (QCD), laser-driven solid state emitters, and earlier Cherenkov FELs are also very limited in output power.
Thus, the problem with existing compact THz sources is low output power, whereas more powerful undulator-based FEL sources (having over kW peak power) are in national facilities that are extremely large and expensive. Undulator-based sources are very inefficient in the specific 0.3-1 mm wavelength range (between FEL and FEM).